Electrohydraulic power-assisted steering systems of the generic class are well known. A servo steering valve is directly or indirectly coupled with a vehicle steering wheel and put under hydraulic pressure. On torsion, the hydraulic pressure is guided to one or the other side of a steering gear and provides steering power assistance. The hydraulic pressure is produced by a hydraulic pump, which is driven by an electric motor.
In principle, it is desirable in power-assisted steering systems to adjust the steering behavior as a function of the vehicle speed. Prior art describes methods and devices for selectively controlling the steering behavior of power-assisted steering systems. For example, methods are known for controlling the valve characteristic as a function of the steering rate in that the volumetric flow of the hydraulic pump to the steering valve is selectively varied. The reduced volumetric flow can be adjusted, for example, by an interior bypass valve that opens a bypass from the delivery side to the return flow side with increasing speed of the hydraulic pump.
For power-assisted steering systems of the aforementioned type the course of the servo assistance has a narrow valve characteristic for high volumetric flow rates. The driver then finds the power-assisted steering easy. At smaller volumetric flow rates, servo assistance is reduced and the driver finds the steering stiff.
Varying the displaced volume in order to control the valve characteristic is very simple to implement in electrohydraulic power-assisted steering systems. Changing the motor speed also changes the speed of the hydraulic pump, which at low speeds provides low and at high speeds high volumetric flow rates.
The speed of electric motors is changed simply by changing the input voltage. At constant vehicle voltage, for example, in the automobile industry, the motor speed is adjusted by pulse width modulation. Pulse width modulation produces small current pulses, whereby the length of the current pulses defines the motor speed.
Control systems of the described generic class are a more economical alternative to the control of valve characteristics. The driver, however, perceives the increase in the response threshold as a disadvantage. The response threshold defines the amount of torque at which servo assistance begins. This response threshold, also referred to as dead band, increases with smaller volumetric flow and thus impairs the steering precision of the steering assembly. Within the dead band, the driver is steering without any servo assistance only via the elastic torque rod. This decisively impairs the steering precision of the steering assembly. Desirable is a characteristic like the one known, for example, in hydraulic reaction systems, since the dead band of the valve characteristics in these power-assisted steering systems is kept approximately constant.
For example, in steering systems with hydraulic reaction, the steering characteristic of the steering valve is typically controlled by returning the existing operating pressure to a hydraulic-mechanical converter in the steering valve. Such steering assemblies advantageously exhibit a linear increase in servo assistance as a function of the input torque applied by the driver.
Furthermore, using an electrohydraulic converter makes it possible to set a speed-dependent valve characteristic. Such power-assisted steering systems are distinguished by light servo assistance at high speeds. Due to an increase in torsional rigidity, i.e. reduced relative torsion of the steering edges at a given torque, produced by the hydraulic-mechanical converter, the driver perceives steering stiffness. In the parking range, however, the driver finds steering advantageously easy.
The disadvantage of such steering assemblies with hydraulic reaction is their increased price compared to conventional power-assisted steering due to the required additional components such as the electrohydraulic converter, which is controlled by an electric control unit ECU. In addition, the steering valve design becomes more complex and expensive due to the hydraulic-mechanical converter. The economical standard valve of a conventional assisted-power steering system can no longer be used.
Based on the described prior art, the object of the present invention is to define a method that provides an economical way to control the valve characteristic in electrohydraulic power-assisted steering systems without impairing steering precision.